Conventionally, in electronic devices such as, for example, a server, a router, or a storage product, an electrical signal is transmitted using a backplane or a cable, between respective wiring boards between or in respective electronic devices.
Recently, there is a case in which a differential signal transmission is performed using a differential signal transmission cable in order to transmit a high-speed electrical signal between respective wiring boards. The differential signal transmission cable has a pair of core lines (signal line conductors). A plus side signal and a minus side signal obtained by reversing a phase at 180° are transmitted in the core lines, respectively. Further, a potential difference between the two signals becomes a signal level, and is recognized in the reception side in such a way that, when the potential difference is plus, the signal level is recognized as “High,” and when the potential difference is minus, the signal level is recognized as “Low.”
However, conventionally, when an electrical signal is transmitted to a semiconductor package mounted on the wiring board via a cable, a connector placed on a front end side of the cable is connected to a connector on the wiring board, and the electrical signal is transmitted through a wiring pattern of the wiring board. Consequently, the mismatching of impedance may be caused, for example, at a contact point of the connector to increase a transmission loss. Further, since the electrical signal is transmitted through the wiring pattern of the wiring board, the transmission loss may be increased.
The followings are reference documents.    [Document 1] International Publication Pamphlet No. WO 2009/031394,    [Document 2] Japanese Laid-Open Patent Publication No. 10-312863, and    [Document 3] Japanese Laid-Open Patent Publication No. 04-233179.